The present invention is applicable to a two-stage process by which parisons are produced in an injection molding machine, cooled to room temperature, stored and later conveyed in bulk to a reheat, orientation--stretch and blow molding machine. In this manner, oriented hollow plastic bottles are formed. Polyethylene terephthalate (PET) is typical of the plastic material used.
Heretofore cooling of parisons during the first stage has been time consuming, resulting in inefficiency and poor productivity. However, sufficient cooling is indispensable if the parisons are to maintain their shape during later handling operations, Techniques proposed to date for cooling parisons often times created bottlenecks downstream from the injection molds thereby severely hampering productivity. Obviously, a high rate of production of parisons is important in commercial operation, and the rate at which the injection molding cycle can produce the parisons has been limited by the time taken to cool the parisons sufficiently to allow post molding handling without damage to the parison.
Typically, the hollow plastic parisons produced in the injection molding cycle have relatively thick walls and are molded with PET resin at relatively high temperatures. Consequently, after removal from the injection mold, the hollow parison must be cooled sufficiently to prevent deformation or adhesion to one another. However, the thick parison wall tends to trap heat. Cooling the parison in the injection mold by means of its contact with the cool mold surfaces quickly chills the inner and outer skin, but mold cooling will remove the heat trapped within the wall only by keeping the parison in the injection mold for an extended period of time. Naturally, this is economically prohibitive because high parison production is not feasible unless cooling in the injection mold is held at a minimum.
Initial cooling of the molded parison following formation may be sufficient to permit ejection from the injection mold, but additional cooling is then immediately required to remove additional heat as it is conducted to the skin. If the additional cooling were omitted the skin temperature would rise and cause the molded parisons to stick together, to become prone to surface damage, or to bend or warp. Naturally this is unacceptable.
Many approaches have been proposed to alleviate these problems but they either were cost prohibitive, did not optimize injection molding cycles, required excessive parison handling or required cumbersome or highly inefficient dedicated equipment.